The treatment of wastewater and sewage has been developed continuously since medieval times. Since the advent of municipal sewage systems, sewage plants have been built for the treatment of such waste on regional basis. Trickling filters were an early improvement in waste treatment systems. Untreated liquid was spread evenly onto the top of a bed of clinker, coke, broken stone, and other large particles. As the liquid trickled through the bed, oxidation of organic and nitrogenous material in the liquid occurred because of bacteria and other microorganisms which exist on the surfaces of the media of the bed. Solid debris is thus formed which is separated from the liquid in settling tanks. The treatment of sewage wastewater typically requires several stages to remove both solids and soluble and colloidal bio-chemical oxygen demand (BOD). The primary treatment stage is a physical process for removing solids. Wastewater containing raw sewage is passed through a pre-treatment tank where solids settle out by gravity and form a sludge.
A two-step process has been disclosed in several references. Tymoszezuk, U.S. Pat. No. 3,853,752 discloses an improvement in aerobic biological processes for treating waste fluids by combining an activated sludge process with aerated biological filters and non-aerated biological filter beds. These filter beds are similar to presently existing rapid sand filters used for treatment of potable water. Excess sludge is extracted by a sludge concentrator during the first stage.
U.S. Pat. No. 5,006,251 to Takeishi et al., teaches a two-step process for treating organic wastewater. A first step provides the stage precedent to a second step using an aerobic filter bed that employs an upward flow type solid-liquid separator packed with a filter medium. The SS (suspended solids) removal at the preceding stage is performed at a high SS removal rate and in a short time. During the first step, organic wastewater containing SS is introduced into a solid-liquid separator packed with a filter medium having a void ratio of at least 70%, via a lower portion of the separator. The wastewater is discharged from the separator via an upper portion of the separator to remove the suspended solids from the wastewater. A second step introduces the effluent from the first step into an aerobic filter bed packed with a granular filter medium and supplied with oxygen-containing gas from a lower portion of the filter medium-packed bed, via an upper portion of the filter bed. The effluent is discharged from the filter bed via a lower portion of the filter bed to treat the effluent aerobically.
Many attempts have been made to increase the capacity of the filter for a given footprint, that is the area of ground, required by the filter. This is especially important in existing sewage plants which have to increase capacity to meet the demands of a growing population, but cannot increase the area over which the plant extends. The filters have tended to become columns so that a greater filter volume is given by the height of the filter. The upward extent of the filter being limited to the weight, which the base of the filter can support.
As the biological oxidation of the waste occurs on the surfaces of the filtering media, a larger amount of waste can be treated in a given volume of filter by increasing the surface area of the particles in the media. Thus, the average size of the media particles has been reduced in order to increase the surface area of the particles. Furthermore, the particles of the media are often formed from plastics material as this has been found to be particularly advantageous.
It has also been found that the efficiency of the filter is often increased by the filter media being flooded. Flow through the filter may be upwards or downwards. Air is pumped through the flooded filter media bed. It is vital to ensure that the air outlets do not become blocked, otherwise oxidation of the waste would cease. When used with small media (less than about 5 mm nominal diameter) solids are partially retained within the bed of medium and have to be removed by backwashing, requiring costly backwash equipment and downtime for the filter unit. The smaller media and retained particles create higher headloss conditions that may limit treatment capacity. Where solids, are removed by backwashing, the filter is commonly known as a Biological Aerated Filter (BAF). In this case, a settlement tank following the unit is not normally required.
If larger media is used in the filter, solids may not be retained in the bed and are consequently removed by a settlement tank following the filter. In this case where backwashing is not required, the filter is known as either a trickling filter or a Submerged Aerated Filter (SAF). The SAF differs from filter beds (trickling filters) because the media bed is flooded and oxygen is forced into the system from the bottom. U.S. Pat. No. 4,129,374 to Savage, teaches a method and apparatus for the denitrification of wastewater using a facultative zone. The '374 patent involves a wastewater treatment facility that comprises a facultative zone combined with a columnar oxidation unit. In a facultative zone, BOD is solubilized and rendered non-filterable due to low dissolved oxygen conditions thereby impeding the removal of BOD and suspended solids by filtration.